There is a category of English mistake that does not go away with vocabulary, and does not go away with confidence. Someone can write complex, precise, entirely comprehensible English and still produce these, because they are not errors of understanding. They are errors in the parts of the language that have no rule worth learning, only a habit worth acquiring.
Articles are the clearest case. "She is teacher." is instantly understandable, and instantly non-native. The correction is "She is a teacher.". Nothing about the meaning changed. The same applies to "Sun is bright today." becoming "The sun is bright today.", where the article marks something uniquely identified rather than one of many. Many languages have no articles at all, and those that do not simply omit them; there is no partial credit available and no way to reason it out from first principles in the moment.
Prepositions behave the same way and are worse, because they look learnable. "He is married with Sara." becomes "He is married to Sara.". If you ask why that particular preposition, the honest answer is that it is what English does here. Collocation is memorised, not derived, which is why preposition errors persist long after grammar in general has been mastered.
Word order produces a different signature. "I like very much the book." is grammatical in the structure of several other European languages, and the English correction is "I like the book very much.". Nothing is missing from the original; something is merely in a position English does not use for it.
Two more that are pure arbitrary selection. Some English verbs take a gerund and some take an infinitive, with no semantic principle separating them: "I enjoy to swim." is wrong and "I enjoy swimming." is right, while "want" takes the opposite pattern. And auxiliaries fix the form of what follows, so "I have went there." has to become "I went there. (or I have gone there.)".
The pattern that matters for anyone learning is that first-language background predicts the shape of the error, not the amount of it. A speaker of a language without articles omits articles. A speaker of a topic-prominent language may drop a subject that English requires, producing "I bought car yesterday." where English needs "I bought a car yesterday." — English demands a subject even when there is nothing for it to refer to. Knowing which family of error you are prone to is more useful than a general instruction to be careful.
The practical consequence: these are exactly the errors that proofreading yourself does not catch. They do not feel wrong to the person who wrote them, because the sentence means what it was intended to mean. They are caught by something outside your own ear.
A deliberate note on what this post does not contain: no frequency statistics. The reference table behind it ranks these patterns, but that ranking is editorial ordering rather than a corpus study, so quoting it as data about how often learners make these mistakes would be inventing a measurement. The examples and the rules are the sound part, and they are what is used here.
A writing assistant that flags these in place is at belikenative.com.